The present invention relates to an ion sensor suitable for being set in apparatuses for analysis of ion species contained in a biological fluid, for example, blood analyzers for medical use, and more particularly to a solid-state ion sensor which does not need special electrolyte solution and which is small in size and can comprise integrated ion sensors for multi-species.
An ion-selective electrode is characterized by selectively determining concentrations of specific ion species in a solution and has been employed in various fields including monitoring for ion concentration, water analysis, etc. Particularly, in the medical field it is applied to quantitative determination of ion species contained in blood or biological fluids such as urine, etc, for example, chloride ions, potassium ions, etc. Since concentrations of specific ion species in a biological fluids are closely related to metabolic reactions of living bodies, diseases for high blood pressure, diseases for kidney, diseases for neurosystem, etc. are diagnosed by determining concentrations of specific ion species.
In the case of a ion-selective electrode, between an activity a of ion species to be determined and an electrode potential E given by the ion-selective electrode, a correlation that the logarithm of activity a is proportional to a change in the electrode potential E is established as shown by the following equation: EQU E=E.degree.+2.303(RT/Zf) log a
and the activity a of ion species to be determined can be simply calculated from measurements of the electrode potential E. In this way, when an ion-selective electrode is used, the quantitative determination of ion species in a wide range of concentrations becomes possible only by measuring the potential E. In the above equation, R is a gas constant, T is an absolute temperature, Z is a valency, F is a Faraday constant, E.degree. is a standard electrode potential of the system and log is a logarithm.
Generally, as shown in FIG. 15, the conventional ion-selective electrode containing an inner solution comprises sensor body 51 filled with inner solution 52, silver/silver chloride inner electrode 53 being dipped in the inner solution 52, ion-selective membrane 54 being fixed at the center of the sensor body 51 along biological fluid channel 55. In the conventional ion-selective electrode, an aqueous electrolyte solution or an agar gel containing a supporting electrolyte is used as the inner solution 52 which serves to conduct electricity between the ion-selective membrane 54 and the inner electrode 53 ("Analytical Chemistry", Vol.52 No.4 pp.692-700 (1980)).
A small-sized potassium ion sensor manufactured by photolithographic process is described in "Sensors and Actuators", Vol.11 (1987), pp.23-36. This sensor is made by forming a conductive electrode such as of a metal on a silicon chip and providing an ion-selective membrane on the conductive electrode.
Furthermore, an ion-selective electrode is known which has a layer comprising a dry residue of a hydrophilic material provided between an electrode comprising a metal and its water-insoluble salt and an ion-selective membrane (U.S. Pat. No. 4,053,381).
In the above-mentioned conventional solid-state ion sensor described in "Sensors and Actuators", Vol.11 (1987), pp.23-36, the ion-selective membrane is directly provided on the electrode. Therefore, the sensor has the problem that exchange of charges between the ion-selective membrane and the electrode is not efficiently conducted and thermodynamic equilibrium is not established and thus, the potential drift is great. Moreover, since there is no electrolyte between the ion-selective membrane and the electrode, this solid-state ion sensor greatly differs in the electrode potential from the conventional ion-selective electrode having an electrolyte (inner solution) and the measuring circuit for conventional ion sensors cannot be used and a special measuring circuit is needed.
Furthermore, the conventional ion-selective electrode in which an agar gel containing a supporting electrolyte is used as the inner solution which serves to conduct electricity between the ion-selective membrane and the inner electrode has the problem that the stability decreases when used for a prolonged time due to the gradual gasification of water molecules contained in the agar gel.